It's not that common in our tanks, but I had one that set up a cleaning station right at the front corner of my 100g where I could see it best. Angels, tangs & all the fish would slowly come in and wait their turn to get cleaned. It would literally climb all over the fish & the fish would open mouths & gill covers for a thorough cleaning. Any parasites or dead skin was picked off &
It's an awesome sight & a chance to see such symbiotic relationships are one of the main attractions and reasons why a lot of people get into sw (re: clownfish-anemone)
Unfortunately, something killed it a couple of months ago
Hope to have that again in one of my tanks in the future (ps. stay away from hawkfish, some crabs, and bird wrasse who are too stupid to know leave their "doctor" alone.)
Anthony